README about working with code fixes

vsintegration/src/FSharp.Editor/CodeFixes/CodeFixes.md

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+# Code Fixes In F#
+
+Code fixes are suggestions provided by IDEs to automatically address coding issues. In Visual Studio, they fall under the category of [Quick Actions](https://learn.microsoft.com/visualstudio/ide/quick-actions) and are displayed within the light bulb icon menu.
+
+## Quick Facts
+
+1. Code fixes are triggered by one or more [diagnostics](https://github.com/dotnet/fsharp/blob/main/docs/diagnostics.md) (e.g., FS0039 and/or IDE0005).
+1. If there are multiple diagnostics, they are guaranteed to share the same _span_ (the red squiggly).
+1. Every code fix can produce:
+  - No suggestions (got triggered but couldn't come up with a suitable suggestion for a fix).
+  - One suggestion (e.g., "Discard unused value").
+  - Multiple suggestions (e.g., "Open namespace X" or "Fully qualify namespace X").
+
+![Important bits about code fixes](./codefixes-1.png)
+
+_Bulk code fixes_ perform the same actions for identical diagnostics across a document/project/solution:
+
+![Bulk code fixes](./codefixes-2.png)
+
+## Technical Implementation
+
+- Code fixes need to adhere to certain Visual Studio API requirements:
+  - Have the `[<ExportCodeFixProvider>]` attribute.
+  - Inherit from the `CodeFixProvider` class.
+- When inheriting `CodeFixProvider`, a few members should be overridden:
+  - `FixableDiagnosticIds` - lists all diagnostics triggering the code fix.
+  - `RegisterCodeFixesAsync` - has the primary code fix logic.
+  - `GetFixAllProvider` - needed for those with bulk behavior.
+- On top of that, we have our own framework for F# code fixes, found in the `IFSharpCodeFix.fs` file. In that regard, code fixes should implement one of:
+  - The `IFSharpCodeFixProvider` interface - for those generating at most one suggestion (the majority).
+  - The `IFSharpMultiCodeFixProvider` interface - for those able to produce several suggestions.
+- Code fixes within the above framework can and should be unit tested.
+
+## Improving Existing Code Fixes
+
+1. Find the corresponding unit test suite.
+2. Introduce a test featuring the code with the diagnostic alongside the expected code fix behavior (0, 1, or multiple suggestions).
+3. Adopt a TDD approach and modify the code fix logic. Keep in mind that occasionally, issues might originate in the compiler layer.
+
+## Creating New Code Fixes
+
+1. Create a matching unit test suite with both positive and negative cases.
+2. Seek a similar code fix for inspiration.
+3. Craft your new code fix, ensuring compliance with the framework described above.
+4. Just before submitting the code fix, manually double-check that it works in VS - demo videos are welcome. 
+
+## Useful Links:
+
+1. [Code fix tracking issue](https://github.com/dotnet/fsharp/issues/15408) - tracks bugs, enhancements, and suggestions for new code fixes.
+2. [A session from Amplifying F#](https://amplifying-fsharp.github.io/sessions/2023/06/16/) that largely focuses on crafting code fixes.